As the trend toward decreasing the size of microelectronic packages continues, manufacturing challenges are continuously encountered. One manufacturing challenge is the need for reliable lead bonding.
FIG. 1 is a partial cross-sectional elevational view of a typical micro-ball grid array (micro-BGA) package 10. The micro-BGA package 10 includes a die 12 having a plurality of bond pads 14 formed thereon. A spacing layer 16 (typically composed of an elastomer material) is formed on the die 12, and an interposer 18 (typically composed of a dielectric material, such as a polyimide tape) is formed on the spacing layer 16. An adhesive layer 20 is formed on the interposer 18.
The micro-BGA package 10 also includes a plurality of conductive leads 22. One end of each lead 22 is coupled to one of the bond pads 14, and the opposite end of the lead 22 terminates in a ball pad 24 that is coupled to the interposer 18 by the adhesive layer 20. An encapsulating material 26 is disposed over the conductive leads 22 and the exposed areas of the die 12 to seal and protect the leads 22 and die 12 from environmental elements. A conductive bump 28 (typically composed of solder) is formed on each ball pad 24. Finally, a coverlay 30 is formed at least partially over the micro-BGA package 10.
The micro-BGA package 10 may be coupled to an electrical circuit (not shown), such as a printed circuit board, by engaging the bumps 26 with corresponding contact pads on the circuit. Micro-BGA packages of the type shown in FIG. 1 are shown and described, for example, in U.S. Pat. Nos. 5,663,106 and 5,777,379 to Karavakis et al, and in U.S. Pat. No. 5,821,608 to DiStefano et al, which patents are incorporated herein by reference.
FIG. 2 is an isometric view of a partially-constructed micro-BGA package 10A of FIG. 1 prior to the addition of the encapsulating material 26, the coverlay 30, or the conductive bumps 28. Typically, the leads 22 are formed from a sheet of conductive material using standard cutting and etching processes to form a lead array 40. The lead array 40 includes a base 42, the plurality of conductive leads 22, which project from the base 42, and the ball pads 24, which are formed at the ends of the leads 22. Each lead 22 may include a frangible section 44. Processes for forming the lead array 40 are shown and described, for example, in International Patent Publication WO94/03036 published Feb. 3, 1994, or U.S. Pat. No. 5,629,239 to DiStefano et al, both of which are incorporated herein by reference.
The ball pads 24 and leads 22 are engaged with the adhesive layer 20 to couple the ball pads 24 and leads 22 to the interposer 18. Then the leads 22 are bonded to the bond pads 14 of the die 12. A bonding tool 50 is typically used to bond the leads 22 to the bond pads 14. As described in U.S. Pat. No. 5,629,239, the bonding tool 50 moves downwardly toward the die 12 until it engages the lead 22. The bonding tool 50 continues moving downwardly, snapping or breaking the frangible section 44 of the lead 22, and downwardly bending the lead 22 until the lead 22 engages the bond pad 14. The bonding tool 50 then bonds the leads 22 to the bond pads 14 in the conventional manner (e.g. thermally, ultrasonically, etc.).
Alternately, one or more of the leads 22 may be broken by depressing a cutting blade (not shown) against the leads 22, bending the leads 22 downwardly until the frangible sections 44 are snapped or broken, as described in U.S. Pat. No. 5,629,239. The bonding tool 50 is then applied to the lead 22 to continue bending the lead 22 downwardly until the lead 22 engages the bond pad 14 and is bonded into position.
One significant problem attributable to these manufacturing methods, however, is that the downward bending and snapping of the leads 22 during the bonding process can adversely effect the physical and electrical connections throughout the micro-BGA package 10. For example, the physical attachment of the ball pads 24 and the leads 22 with the adhesive layer 20 and the interposer 18 may be weakened or detached during the bending of the leads 22, and during the snapping of the frangible sections 44. Also, after one of the leads 22 is bent and bonded to the corresponding bond pad 14, the physical and electrical connection between the lead 22 and the bond pad 14 may be weakened or detached due to flexure of the micro-BGA package 10 (especially the elastomer spacing layer 16) as one or more adjacent leads 22 are being bent, snapped, and bonded to the associated bond pads 14.
Thus, the bending moments exerted on the leads 22, and the compressive forces of the bonding tool 50 on the leads 22, may be communicated throughout the micro-BGA package 10 as the leads 22 are snapped and bent into engagement with the bond pads 14. These forces and moments may adversely impact the physical and electrical connections between the components of the micro-BGA package 10, resulting in a significant rate of failure.